The present invention generally relates to an OFDM receiver and, more particularly, to a guard interval length detector for OFDM signals and a method of operating the same.
In telecommunications, guard intervals are used to ensure that distinct transmissions do not interfere with one another. These transmissions may belong to different users, as in TDMA systems, or to the same user, as in OFDM systems. Due to the immunity against multipath reflection distortion, OFDM becomes one of the main stream techiniques of communication and digital terristrial broadcasting application nowadays. Digital Audio Broadcasting (DAB) and Digital Video Broadcasting (DVB) are two of the examples, wherein DAB is a technology for broadcasting audio programming in digital form, and DVB is a standard for broadcasting Digital Television over satelite (DVB-S), cable (DVB-C), terrstrial (wireless) transmission (DVB-T), and handheld device (DVB-H). Integrated Service Digital Braodcasting-Terrestrial (ISDB-T), the digital television and digital audio broadcasting format that Japan has created, is another example of the applications adopting OFDM scheme. Moreover, the applications that adopt OFDM technology still comprise Asymmetric Digital Subscriber Line (ADSL), Very-high-speed Digital Subscriber Line (VDSL), Wireless Local Area Network (WLAN) including IEEE 802.11a/g/n, Ultra Wideband (UWB), and Dedicated Short Range Communications (DSRC) systems.
The purpose of the guard interval (GI) is to introduce immunity to propagation delays, echoes, reflections, inter-channel-interference (ICI) and inter-symbol-interference (ISI), to which digital data is normally very sensitive. DVB-T defines GI in its specification in order to resist the ISI resulted in the multi-path. Part of the data is duplicated and placed in front of the transmission data packets to be the cyclic prefix. For DVB-T system, there are two modes of effective data length, 2K and 8K, and four different guard interval (GI) lengths, 1/32, 1/16, ⅛ and ¼ of the data length, represented by GI= 1/32, 1/16, ⅛ and ¼, respectively.
The duplicated GI needs to be removed before the receiver process the data in order to perform the Fast Fourier Transform correctly in accordance with the data length, that is, 2K elements or 8K elements. There have been several methods and related circuits proposed in the prior art to detect the mode of the received OFDM signals and the GI length by performing correlation to the magnitudes of the received signals, taking advantage of the inherent quality of the guard interval. However, the mode detection performed by these convnetional methods and circuits mentioned above is often susceptible to noise, channel effiect and sampling frequecy offset. Hence a normalization is usually required in the prior art. Otherwise, it is often not easy for these methods and devices proposed in the prior art to define the threshold of the correlation of the magnitudes. There are still methods proposed in the prior art to perform correlation to the phases of the received signals. However, these methods in the prior art detect only the beginning of packets but not the transmission mode.